Some statistics are in Republicans' favor. For instance, Donald Trump won the district by 16 points in the 2016 presidential election.

The National Republican Congressional Committee is well aware of this and is already prepping its candidate, Pete Stauber, a St. Louis County commissioner and a police officer.

"We currently have one of the strongest recruits in the country, Pete Stauber, in the race who's garnering strong local support,” said NRCC communications director Matt Gorman.

Nolan, however, hopes his early exit will give Democrats a chance to preen their own capable contender.

"Hopefully the timing of this announcement in this still-young election year will provide prospective candidates with ample opportunity to present themselves to voters and activists at the many party county conventions to be held over the next several months throughout our region," Nolan said in a statement.

Nolan first entered Congress in 1974. He briefly retired ahead of the 1980 elections, before running another successful campaign in 2012.

The Democrats' once double-digit advantage on the generic congressional ballot has decreased to just 6 points, thanks in large part to voters crediting the GOP for the improving economy after they passed tax reform.